Alkali metal production

ABSTRACT

WHICH ARE REMOVED OVERHEAD. THE REDUCIBLE ALKALI METAL COMPOUND MAY BE AN ALKALI METAL CARBONATE, AN ALKALI METAL HYDROXIDE, AND AN ALKALI METAL COMPOUND (SUCH AS BORAX) HAVING A NEGATIVE GROUP CONTAINING AN AMPHOTERIC METAL WHICH LATTER COMPOUND PROVIDES UPON REDUCTION THE ALKALI METAL VAPOR AND THE AMPHOTERIC METAL IN A REDUCED SOLID ELECTRICALY-CONDUCTIVE FORM. THE LATTER ALKALI METAL-AMPHOTERIC METAL COMPOUNDS IN PROVIDING ELECTRICALLY-CONDUCTIVE SOLID RESIDUE UPON REDUCTION ARE PROCESSIBLE IN BEDS OF VARYING TYPES.   METHOD FOR PRODUCING ELEMENTAL ALKALI METAL FROM A REDUCIBLE ALKALI METAL COMPOUND IN A CARBOTHERIMIC REDUCTION FURNACE INVOLVING, IN A PREFERRED EMBODIMENT, PASSING A DOWNWARDLY MOVING BED THROUGH A HEATING ZONE OF THE FURNACE WITH A REDUCIBLE ALKALI METAL COMPOUND AND A CARBON REDUCING METAL DISPERSED IN THE BED IN PARTICLES SMALLER THAN THE ELECTRICALLY-CONDUCTIVE BALLS WITH THE REDUCIBLE ALKALI METAL COMPOUND COMPRISING ON A WEIGHT BASIS LESS THAN 50% OF THE BED, HEATING THE BED WITH ELECTRIC CURRENT TO A TEMPERATURE IN THE RANGE OF ABOUT 900*C. TO ABOUT 1500*C. TO EFFECT REDUCTION OF THE ALKALI METAL COMPOUND TO PRODUCE ALKALI METAL VAPORS

Oct. 3, 1972 H. B. H. COOPER ALKALI METAL PRODUCTION 2 Sheets-Sheet 1 Original Filed Dec. 28. 1966 wN m HAL B.H.COOPER INVENTOR.

1972 H. B. H. COOPER 3,695,864

ALKALI METAL PRODUCTION Original Filed Dec. 28, 1966 2 Sheets-Sheet z I no g Q I 11 5 a i (3% 9 \l L'u D: D Q) L 0 cm -J INVENTOR.

HAL B. H. COOPER T. REID ANDERSON ATTORNEY United States Patent Oflice US. C]. 75-66 9 Claims ABSTRACT OF THE DISCLOSURE Method for producing elemental alkali metal from a reducible alkali metal compound in a carbothermic reduction furnace involving, in a preferred embodiment, passing a downwardly moving bed through a heating zone of the furnace with a reducible alkali metal compound and a carbon reducing metal dispersed in the bed in particles smaller than the electrically-conductive balls with the reducible alkali metal compound comprising on a weight basis less than 50% of the bed, heating the bed with electric current to a temperature in the range of about 900 C. to about 1500 C. to effect reduction of the alkali metal compound to produce alkali metal vapors which are removed overhead. The reducible alkali metal compound may be an alkali metal carbonate, an alkali metal hydroxide, and an alkali metal compound (such as borax) having a negative group containing an ampheteric metal which latter compound provides upon reduction the alkali metal vapor and the amphoteric metal in a reduced solid electricaly-conductive form. The latter alkali metal-amphoteric metal compounds in providing electrically-conductive solid residue upon reduction are processible in beds of varying types.

This is a continuation application of patent application Ser. No. 613,373, filed Dec. 28, 1966, now abandoned, which, in turn, is a continuation-in-part application of Ser. No. 498,061, filed Oct. 19, 1965, now abandoned.

The present invention relates to the production of alkali metals and more particularly to an improved carbothermic process for the reduction of alkali metal compounds and to advancement in the protection of the alkali metal products against loss during recovery from an efiiuent reactor stream. The carbothermic process of the invention in some embodiments provides for the simultaneous production of carbides or reduced metal forms of various amphoteric metals, along with the alkali metals.

Substantially all sodium and related alkali metals produced commercially today are obtained by fused salt electrolysis of sodium chloride and other suitable alkali chloride materials. Prior to the early 1900s elemental sodium was produced by carbothermic reduction of sodium carbonate. The carbothermic process was plagued with severe engineering and operating shortcomings, and with the advent of fused salt electrolysis the carbothermic route was abandoned. Nevertheless, the high electrical energy and capital investment requirements of the conventional electrolysis method have prompted over the years a considerable research effort to develop a more economical process for the production of sodium. Interest has continued in carbothermic reduction but to this time such carbothermic processes as have been proposed are typified by low yields and mechanical shortcomings.

The carbothermic reduction of alkali metal compounds required a large input of heat and the reactions proceed only at high temperatures. For example, the carbothermic reduction of sodium carbonate may be carried on at tem peratures as high as 1500 C. When heat is supplied for 3,695,864 Patented Oct. 3, 1972 this highly endothermic reaction through the walls of a reactor furnace, large heat losses are inevitable. The furnace walls deteriorate rapidly because of the extreme corrosion caused by the fused mass at the inner faces of the walls and because of the extreme high temperatures which must be maintained on the faces of the furnace Wall to provide the required reaction temperature within the furnace.

In the carbothermic reduction of sodium and other alkali metal compounds, the efiiuent stream from the reactor furnace contains alkali metal vapor and a large quantity of carbon monoxide. Reversion reactions if not carefully guarded against will reduce significantly the yield with sizeable amounts of the alkali metal vapor returning to the carbonate or oxide form.

Various procedures have been suggested for the carbothermic approach for increasing the yield of the alkali metals by preventing reversion reactions. The proposed techniques have generally recognized the necessity of quickly cooling the reaction products of the carbothermic process to a relatively low temperature, i.e. to a temperature below which the reversion reactions occur at a significant rate. Among the proposed. procedures have been the use of large cooling surfaces and the employment of large volumes of cooling gas which are precipitously mixed with the efiluent stream from the carbothermic reactor. Other suggested approaches utilize molten tin or molten lead. In the use of the molten metal coolants, the efii-uent gas is exposed to a shower of the molten lead or tin with the result that the efiiuent gases are rapidly cooled to below the temperatures at which the objectionable reversion reactions proceed and simultaneously the sodium or other alkali metal vapor is condensed and forms an alloy with the metal liquid coolant. The sodium or other alkali metaltin or lead alloy is then distilled under a vacuum to recover the alkali metal product. It will be appreciated that the latter method necessitates the handling of large quantities of molten metal, requires considerable power to pump the metal liquid coolant, and needs additional heat to separate the alkali metal from the alloy.

It is a principal object of the invention to provide an improved carbothermic reduction process for alkali metal compounds which process does not involve the transfer of large quantities of heat through a furnace wall.

It is a still further object of the invention to provide an improved process for the carbothermic reduction of alkali metal compounds which minimizes corrosion problems and accordingly lessens maintenance.

It is a still further object of the invention to provide a carbothermic process for the production of sodium and other alkali metals which is more eflicient than heretofore proposed carbothermic reduction processes and which requires less energy and capital investment than conventional fused salt electrolytic processes.

It is a still further object of the invention to provide a carbothermic process having a high alkali metal yield.

It is a still further object of the invention to provide an improved means of minimizing reversion reactions in the condensing step of a carbothermic sodium or other alkali metal recovery process.

It is another object of the invention to provide a carbothermic reduction process for producing an alkali metal utilizing a moving bed made up of electrically conductive carbon-containing particles.

It is another object of the invention to provide an improved process for separating alkali metal vapors from a gaseous stream.

It is another object of the invention to provide a carbothermic process for the recovery of alkali metal wherein the presence and attending disadvantages of a large, continuous, highly-corrosive, high-temperature fused mass are avoided.

It is a still further object of the invention to provide in one embodiment of the invention a fluidized bed employing at least in part high temperature refractory, electrically conductive, carbide particles.

It is a still further object of the invention to provide a carbothermic reduction process for producing an alkali metal utilizing a fluidized bed wherein a reducible alkali metal compound such as borax is continuously introduced to the bed along with a reducing agent such as carbon or a carbonaceous gas.

It is another object of the invention to provide a carbothermic fluidized reduction process wherein a reducible alkali metal compound mixed with carbon in a pellet or particulate form is continuously introduced into the bed, with the carbon being present in at least a stoichiometric amount for reaction with the reducible compound.

It is a further object of the invention to provide a method for the reduction of an alkali metal compound having a negative group containing an amphoteric metal, for example, boron, chromium, aluminum, silicon, vanadium and tungsten, to simultaneously produce the alkali metal as a vapor and the amphoteric metal in a reduced electrically-conductive solid form, typically as the element and/or metal carbide.

Heretofore carbothermic reduction processes required the high temperature heating of large molten masses of reducible alkali metal compounds such as sodium carbohate and a reducing agent, typically carbon. It will be appreciated that the mere mechanical handling of such high temperature molten masses presents many problems apart from the low efiiciency and low yields characteristic of such processes. It has now been found that alkali metals may be produced in high yields and with high efficiency by utilizing a moving or agitated bed of electrically-conductive particles (the particles are generally present in a predominant amount) to heat the reducible alkali metal material and a carbon reducing material. In one embodiment of the process, the electrically-conductive particles and carbon reducing material are the same composition. An electric current is passed through the bed heating it to a temperature in excess of about 900 C., thereby accomplishing reduction of the alkali metal material to its corresponding alkali metal. The moving bed is kept at a temperature sufficiently low to avoid significant volatilization and loss of the reducible alkali metal material in the eflluent gas stream from the bed. The alkali metal vapors are subsequently recovered from the eflluent stream.

Generally the bed will be maintained at a temperature in the range of about 900 C. to about 1500 C. and preferably in the more narrow range of about 900 C. to about 1200 C. The temperature range employed is such that carbon dioxide is reduced to carbon monoxide, yet low enough so that, for example, sodium carbonate or sodium oxide, sodium borate, or other alkali metal reducible materials present are not volatilized from the bed without reduction. At temperatures above 900 C. the carbon dioxide-carbon monoxide equilibrium in the presence of excess carbon favors carbon monoxide and at temperatures below about 800 C. the equilibrium shifts to carbon dioxide.

Usually the electrically conductive carbon containing particles of the bed will be elemental carbon, preferably petroleum coke, but in some applications of the process of the invention, for example in the carbothermic reduction of borax or other alkali metal-amphoteric metal compound, even where the bed is initially wholly carbon particles, there will be a buildup of reduced boron and boron carbide particles (or other amphoteric metals and amphoteric metal carbides) which are even more electrically conductive than carbon. In some applications it may even be desirable to initially commence the operation with a wholly carbide particle bed and continuously introduce the reducing agents, say, carbon particles or a reducing gas, to the bed along with the reducible alkali metal material being processed.

Typically, in the operation of the process of the invention the petroleum coke particles or other electrically conductive particles will be present in the moving bed in a predominant amount with the reducible alkali metal compound particles or pellets being present in a minor amount. With a minor amount of the reducible alkali metal compound present, the bed does not become a fused mass, interfering with the electrical conductivity, nor suffer the corrosion problems characteristic of carbothermic reduction processes of the past. Desirably, the reducible alkali metal material is present in the bed in an amount less than 50% on a weight basis, usually less than 30%, and pref-' erably, in an amount within the range of 20% to 5%. In some applications of the process of the invention, for example, in the reduction of sodium carbonate or sodium hydroxide, the electrically conductive particles of the agitated bed and the carbon reducing material are one and the same, preferably petroleum coke. The sodium containing material may be introduced separately or in admixture with the carbon and in one modification the sodium carbonate or other reducible alkali metal material is supplied to the bed in a pellet or particulate form which pellets also contain carbon. The carbon supplied to the bed for reduction purposes is preferably provided in a stoichiometric or near stoichiometric amount to react with the sodium carbonate and thus maintain the carbon content of the bed more or less constant.

The electrically conductive bed may be either a fluidized bed of electrically conductive carbon containing particles or in an alternative form, fusion of the bed is avoided by introducing to an upper level of the reactor furnace, relatively large sized carbon containing balls or pellets which descend under the influence of gravity through the bed with the reducible alkali metal material and the carbon reducing agent being supplied in smaller particle size. The balls are of sufiicient mass relative to the more finely divided particles or pellets of alkali metal compound and active reducing agent, to assure a gradual descent of the bed through the electrical heating zone. The balls have sufiicient contact with each other to supply a path for passage of the electric current, yet allowing the alkali metal vapors and carbon monoxide formed in the reaction to pass freely through the bed and leave the reduction reactor.

Where the fluidization technique is utilized, the bed of electrically conductive particles is fluidized by gas which may be recycled carbon monoxide, hydrogen, volatile hydrocarbons, nitrogen, mixtures of the foregoing, or other gases which will not interfere with the progress of the reaction. Among the various hydrocarbon gases that may be utilized are methane, ethane, propane, butane, ethylene, propylene and natural gas. Volatilized normally liquid hydrocarbons, e.g., benzene, aromatic and naphthenic streams, may also be used.

Where the fiuidizing gas is made up in part of a hydrocarbon, the hydrocarbon gas will be thermally cracked under the high temperature of the bed and provide carbon for the reduction of the alkali metal material being processed. Hydrocarbon gas will generally provide only a part of the needed reducing capacity with the rest being supplied by the electrically conductive petroleum coke or other carbon particles. However, where electrically conductive particles of a fluidized bed are at least in part refractory carbide material, the reducing capacity may be conveniently supplied by introduction of hydrocarbon gas to the bed.

The fluidized bed carbon particles normally range in size from 8 to 200 mesh US. Standard Sieve. It will be appreciated that particles outside the foregoing range are sometimes present and will not interfere with fluidizing of the bed. The pellets or particles of the alkali metal material being introduced to the fluidized bed will generally be within the range of 20 to 200 mesh US. Standard Sieve.

Generally the alkali metal particles will not be significantly larger than the size of the fluidized bed particles; otherwise, they would tend to settle to the bottom of the bed. The particle size of the petroleum coke or the alkali compound being processed should not be too small; otherwise there is a tendency for the particles to be blown and carried out of the furnace reactor by the gas stream.

Generally speaking, relatively low entering fluidizing gas velocities, typically less than one foot per second are employed soas to minimize entrainment of carbon in the effluent stream from the bed. There is a major increase in gas velocity as the reduction reaction proceeds, owing to the production of alkali metal vapors and carbon monoxide. The depth of the bed will depend among other things upon the material being reduced, the reducing agent employed, and the nature of the electrical conductive particles. Selections of mesh sizes, fluidizing gas velocities, depth of bed and the like, may be made in accordance with known fluid bed practices.

In the alternative form of the bed, the relatively large carbon containing electrically conductive balls will generally be formed of a highly densified carbon or carbide material, such as boron carbide. The balls will be significantly larger than the particles of the alkali metal compound and reducing carbon. The alkali metal compound and carbon may be separately supplied or in an alternative, supplied in a compact particulate form which contains both the carbon and the reducible alkali metal compound. Here as in the fluid bed approach, the reducing carbon is preferably provided in a stoichiometric or near stoichiometric amount to react with the alkali metal compound. The alkali metal compound and carbon particles will generally be supplied in a size within the range of about 4 to about 200 mesh US. Standard Sieve. The electrically conductive balls will typically have a maximum dimension or diameter of A to 3 inches. It will be appreciated that the balls and particles may take different shapes and sizes, but will be generally spherical. The balls may be formed of various electrically conductive material, including densified carbon, boron carbide, and other refractory carbides, for example, silicon carbide.

Pellets or granules containing both the alkali metal compound and the carbon may be prepared by sinter pelletizing. In one approach carbon and the alkali metal compound are introduced in particle form to a gas fired rotary kiln, which is maintained at a temperature somewhat in excess of the melting point of the alkali metal compound. For example, in the instance of borax, the kiln is heated to a temperature somewhat in excess of 755 C. Alternatively, the carbon particles may be preheated to an elevated temperature of, say, about 800 C. then introduced with the alkali metal compound to a tilted, rotating cylindrical vessel where the heated carbon partially melts the borax and forms integrated pellets of the two materials. The integrated pellets may be used in either the fluid bed or the descending bed technique. Integrated pellets may also be produced with a compaction mill or rolls to compress the alkali metal compound and reducing carbon together.

In the practice of the downwardly-moving bed alternative, the bed descends as material including electrically conductive balls is removed from the base of the furnace reactor. With the reactant system of the alkali metal compound and active reducing agent being interspersed through out the downwardly descending bed of electrically conductive balls, the balls precent formation of a large, fused, unworkable mass which would otherwise result. The alkali metal compounds generally melt at temperatures lower than that required for the carbothermic reduction. The balls provide in a sense a vehicle for the downward passage of the reactants through the furnace.

In a modification of the descending bed technique, the materials are supplied to the heating zone in an integrated briquette form which contains the carbon reducing agent, the reducible alkali metal compound and the electrically conductive carbon material. It will be appreciated that the carbon reducing agent and the electrically conductive carbon material may be one and the same, for example, petroleum coke. In some applications, it will be found more beneficial to employ a carbide material such as boron carbide, silicon carbide and the like as the electrically conductive carbon material. In the latter instance, the carbon reducing agent will generally be supplied in more than a stoichiometric amount for reaction with the reducible alkali metal compound of the briquette. It will be appreciated that if the briquette is essentially a two component particle, that is made up of carbon and the alkali metal compound, it is necessary to provide the carbon in a considerable excess over and above that required for reaction with the reducible alkali metal compound. If, in the: latter two component briquettes, the carbon were to be supplied only in a stoichiometric amount, there would he no material to serve as a blotter for the melted borax and the bed would tend to fuse into a substantially unworkable molten mass. The integrated briquette may be prepared with the sintering technique described above. Alternatively, an organic bonding agent, such as tar, heavy oil, or asphalt may be used for forming of the briquette.

The rate of descent through the furnace reactor is such that substantially all of the alkali metal compound reacts by the time the succeeding strata of the bed reach the lower end of the furnace reactor. At the bottom of the furnace the electrically conductive balls remain in admixture with excess carbon in the instance of a sodium carbonate reduction or, in the case of a borax reduction, there remains boron, carbon and boron carbide in admixture with the electrically conductive balls. A small amount of stripping gas may be introduced to the lower end of the descending bed reactor to insure the passage of all sodium vapors out of the top of the furnace. The stripping gas may be recycled carbon monoxide, an inert gas, or a volatile hydrocarbon gas. The hydrocarbon gas when used also serves as an active reducing agent; however, the cost of carbon from natural gas is generally considerably in excess of that from a source such as petroleum coke. The hydrocarbon gas may, however, be advantageously used for completing the last. stages of the reduction reaction where it is important to minimize the amount of carbon in the reduced metal product recovered from the furnace, for example, in the reduction of borax to produce sodium and reduced boron.

The electrically conductive balls including the single component densified carbon balls and the two or three component balls described above may be prepared by a high pressure extrusion molding technique of the general type described in US. Pat. No. 3,254,143, Heitman.

Where alkali metal carbonate is the source of the alkali metal and there is no solid product out of the bottom of the furnace reactor, densified carbon is the more practical material for forming of the electrically conductive balls. In instances, however, where the reduced metal or metal carbide product out of the bottom of the furnace reactor is valued, then a somewhat higher purity product may be obtained by using balls made of the carbide of that metal. For example, in the case of borax, where boron and boron carbide are recovered as products, an advantage is to be had in using boron carbide electrically conductive balls.

Heretofore, suggested procedures for the recovery of the metallic sodium vapors from the efiluent gas stream of the furnace reactor have resulted in generally low sodium metal yields. In the process of the invention, the gases issuing from the furnace reactor are immediately quenched to avoid reversion reactions which would result in the formation of alkali metal compounds, leading to a proportionately less yield of the alkali metal. For example, in the carbothermic reduction of sodium carbonate the improved condensation technique of the invention minimizes reversions back to sodium carbonate, carbon, carbon dioxide, and sodium oxide. Additionally, the system of the invention shields the condensed alkali metal from subsequent reactions. It is important in the obtaining of a high alkali metal yield that the temperature of the effluent gases be rapidly lowered. An understanding of the chemistry involved will be appreciated by a review of the reactions set forth below. For example, the carbothermic reduction of sodium carbonate to produce elemental sodium is expressed by the overall equation:

The reaction presumably occurs in three stages as follows:

The reaction is highly endothermic 298=23lK cal./g. mole) and it will thus be appreciated why the considerable amount of energy that must be supplied at the high temperature level of the reaction leads to difficult corrosion problems in the prior art processes where large fused masses engage the walls of the furnace reactors.

Above 900 C. the carbon dioxide-carbon monoxide equilibrium in the presence of excess carbon shifts to favor the formation of carbon monoxide. Below about 800 C. the equilibrium reverts to carbon dioxide. The presence of carbon dioxide in the effiuent stream from the furnace reactor must be avoided, as is apparent from the reversion reactions set forth in Equations 5, 6, and 7 below.

In order to prevent or minimize formation of carbon dioxide it is important to quickquench and freeze the products of the favorable high temperature equilibrium in order to obtain good yields of the elemental sodium or other alkali metals.

The quickquench technique of the invention not only assures high yields of the alkali metal but it also serves to isolate and shield the elemental metal as it condenses from the vapor. The efiluent gases are rapidly cooled from their elevated temperature in excess of generally lO- C. to a temperature below which the reversion reactions occur, if at all, at an insignificant rate. The effluent gases should be quickquenched to a temperature less than 400 C. and desirably to less than about 300 C.

In the improved process of the invention a stable, nonreactive, organic material, usually a high boiling, normally-liquid hydrocarbon oil having in the instance of sodium recovery a boiling point in excess of 105 C., preferably above 125 C., e.g., kerosene or related petroleum fractions, or a normally-solid hydrocarbon material such as 'biphenyl, naphthalene or various of their derivatives, may be used as the quenching liquid. Quenching is desirably achieved by a splashing technique, wherein a pool of the stable hydrocarbon liquid is subjected to a high state of turbulence by agitators or splashers in a closed vessel into which the hot, sodium-containing gas stream flows from the furnace reactor. A spraying or other technique may also be used for bringing the hot gases and hydrocarbon liquid into contact. For example, the hot gases are contacted by a spray of the hydrocarbon liquid falling downward into a pool of the oil, so as to remove and protect the alkali metal. The hydrocarbon oil in the amount employed quickly quenches the hot gas stream down to a temperature less than 400 C. and desirably within the range of 250 to 100 C. in the instance of sodium. The elemental sodium or other alkali metal is condensed to its liquid form (melting point of 97.5 C. for sodium) and collects beneath the oil coolant at the bottom of the spray-quench vessel from where it is recovered and subsequently separated from the oil. The alkali metals other than lithium have somewhat lower melting points than sodium, for example, potassium 634 C., cesium 28.7 C. and rubidium 38.8 C. and consequently gas streams containing the latter alkali metals may be cooled to lower temperatures, that is, to temperatures somewhat above their respective melting points. Lithium melts at 180 C. The gas stream will generally be cooled to less than 250 C. and at least 5 to 10 C. above the melting point of the alkali metal being condensed. The hydrocarbon or other organic coolant should have a boiling point of at least 5 C. above the melting point of the alkali metal being condensed and preferably at least 25 C. above. It is important for the subsequent separation of the condensed alkali metal that the liquid in the spray-quench vessel be at a temperature above the melting point of the alkali metal being recovered.

The organic liquid pool shields the alkali metal against reaction with any carbon dioxide that may be present in the gases. The carbon monoxide containing gases are then passed to a heat exchanger where the quenching liquid is condensed and heat recovery may be obtained. The carbon monoxide is then either exhausted to the atmosphere, burned for heat recovery, recycled as a fluidizing gas to the fluidized reactor for preheating the feed solids, or as a heat source in the sinter pelletizing operation.

There are many high boiling hydrocarbon oil mixtures, such as petroleum distillation fractions, that may be employed in the condensing or quenching step. In addition, there are numerous individual organic compounds for example, high boiling ethers such as dibutyl ether and still higher ethers that can be employed. The major criteria being that they be chemically stable and not be carried away in excessive amount by the existing gas stream. They may be aliphatic, naphthenic or aromatic in nature.

The process of the invention is especially suitable for the carbothermic reduction of alkali metal carbonates including potassium carbonate and lithium carbonate in that all of the compound is substantially recovered from the furnace reactor as carbon monoxide and alkali metal vapor, there being no buildup of a reduced element within the bed. Similarly, in the processing of the alkali metal hydroxides there is no buildup of a reduced component. The process is also valuable for the reduction of other alkali metal compounds including the various salts such as borates, chromates, silicates, vanadates, tungstates and the like. With the reduction of the latter materials, in addition to obtaining elemental alkali metal overhead, carbides or the reduced elements of the negative group remain behind in the bed in a non-volatile solid form, for example B C, Cr C Sic, VC, Al C B, Cr, Si, and V, and the like. It will be appreciated that where the carbrdes or reduced elements may be employed in further processing and carbon contamination is not a problem, the carbothermic bed reduction process of the invention offers additional important economic advantages, for example, in producing the respective halides of boron, chromium, silicon, and aluminum.

The carbothermic bed reduction process of the inventron is readily adaptable and useful for reducing the corresponding alkali metal compounds based on lithium, potassium, rubidium, and cesium as well as sodium. Their relatively low boiling points and melting points also facilitate ready processing, collecting, and refining.

In the drawings:

FIG. 1 is a flow diagram of a preferred practice of the process of the invention employing a fluidized bed; and

FIG. 2 is a flow diagram of another embodiment of the process of the invention utilizing a downwardly-moving bed.

For the sake of convenience, the process of the invention will be described in FIG. 1 with reference to a reduction of sodium carbonate to recover metallic sodium. It will be understood that the process may be used to recover sodium from other reducible materials such as sodium tetraborate (borax), sodium metaborate, sodium hydroxide, sodium silicate, sodium vanadate, sodium tungstate, and sodium chromate as well as other alkali metals from their respective reducible salts.

A feed mixture of sodium carbonate and carbon is introduced via a line 12 under pressure into a charging vessel which serves as a fluidized feed hopper. The sodium carbonate and carbon mixture is transported in a fluidized dense phase through feedline 18 to a fluid bed reactor 14. The fluidizing conveying gas which enters the charging vessel 10 through line 11 may be recycled car-. bon monoxide, a hydrocarbon gas, or an inert gas. The outlet nozzles of feedline 18 open into the fluid bed reactor 14 and are provided with water jackets to forestall excessive heating of the alkali metal compound and thus prevent its fusion therein before the material is dispersed into the fluidized bed for reaction.

The mixture introduced to the fluid bed normally contains a stoichiometric amount or slight excess of carbon. The relative amount of sodium carbonate to the total amount of carbon in the bed is kept relatively low, preferably less than so as to avoid the possibility of the molten inorganic salts fusing the fluidizing bed. Petroleum coke particles are desirably used as the fluidizing material. It will be appreciated that the petroleum coke particles serve a dual role as the electrical conductive particles as well as providing reducing capacity. The energy required for maintaining the fluidized bed at its desirable high temperature of 1050-1150 C. is provided by the electrodes 16 formed of high temperature graphite, silicon carbide, boron carbide or other suitable refractory electrode material. Fluidizing gas, preferably recycled carbon tmonoxide gas, is supplied to the bottom of the bed through a plurality of ports by a line 19. The agitation of the fluid bed assures rapid mixing and dispersing of the feed mixture throughout. Massive fusion of the bed is avoided primarily because of (l) the rapid dispersion of the feed mixture throughout the bed, (2) the generation of the heat at the many diverse locations where the reaction is taking place and (3) the rapidity at which the reaction takes place. The ready availability of the reducing carbon and the excellent heat transfer from the conductive particles to the reducible alkali metal material minimizes fusion and the reduction reaction proceeds rapidly and efficiently. The uniformity of temperature of the fluidized bed and the generation of heat within the body of the bed avoids the higher temperatures and corrosion attacks on the walls of the reactor furnace and heat transfer surfaces which were experienced in the past when heat Was passed though the walls into a fused continuous mass. The temperature is generally best maintained in the range of 1050-1150 C. for sodium carbonate reduction. Higher temperatures result in unnecessary corrosion and energy consumption. It should be borne in mind, however, that the temperature should be sufliciently high that substantially all carbon dioxide is converted to carbon monoxide before it leaves the bed.

The fluid bed reactor 14 illustrated in the drawings is provided with a large overhead disengaging space 20 which promotes separation of solids from the gas stream and their return to the bed below. The sodium-carbon monoxide efiiuent stream passes from the overhead disengaging space 20 of the fluidized bed and is led, with as little heat loss as possible, through a vapor line 24 to a spray condenser 25. In the condenser 25 a stable high boiling hydrocarbon oil, such as a high boiling petroleum distillate is sprayed downwardly into the sodium-carbon monoxide gas stream with considerable turbulence toward a pool of the liquid at the bottom thereof. The vapors are quickly cooled to about 250 C. or somewhat less, whereupon the sodium condenses and collects in the liquid of the pool.

The gases including any volatilized oil are removed from the spray condenser 25 for cooling in an overhead condenser 28 from which a portion of the gases are recycled to fiuidize the bed and the excess gases removed in an exhaust line 4. Makeup oil is supplied to the spray condenser 25 via line 42. The temperature of the liquid pool of oil and alkali metal may be regulated by circulation through pump 26 and cooler 27.

A special feature of the invention is the system provided for separating the liquid sodium or other liquid alkali metal and hydrocarbon oil as well as any sodium carbonate or other solids and carbon that may be carried over and collected in the pool of liquid of the spray condenser 25. Liquid from the spray condenser is removed via a line 29 to a solid-liquid filtration, preferably a heated basket centrifuge 30 which has its inner surface coated with a petroleum coke. A pre-coat petroleum coke slurry is introduced into line 29 immediately preceding the basket centrifuge 30 via a line 31. The slurry is formed of the same oil as the quenching liquid and a pumpable amount of the coke, e.g. 1030% by Weight, in suspension therewith. The liquid stream from the spray condenser 25 enters the interior of the basket and, in passing through the wall thereof, the solids contained therein are filtered out and retained. These solids, which are made up principally of solid sodium carbonate and carbon, are withdrawn from the central bottom of the basket centrifuge and collected in a bin 33 from whence they may be recycled via closed conveyer 43 for subsequent reintroduction to the fluid bed reactor 14. The liquid made up of hydrocarbon oil and liquid sodium passes through the basket of the centrifuge 30 and is removed at the outer periphery thereof through a line 32 and passed to a vessel 34. The stream out of the centrifuge is typically at a temperature of about 150 C. The oil-sodium mixture within the vessel 34 may be further cooled or adjusted to a temperature of about C. by circulation through a heat exchanger 36. Liquid from the vessel 34 is passed via line 35 to a solid bowl centrifuge 38 where the stream is divided into a metallic sodium product stream 40 and hydrocarbon oil stream 42. The heat exchanger 36 may be operated at a temperature to solidify the alkali metal but generally the alkali metal will be separated in its liquid form. The oil is returned to the spray condenser 25 through line 42, or it may be sent to a cleanup still.

The contents of the fluidized bed may be removed in whole or part through a line 17. This provision will be more commonly used in the processing of materials such as the sodium borates which build up a non-volatile residue within the bed in time.

An alternative system for the practice of the process of the invention is illustrated in the flow diagram of FIG. 2. Here a feed mixture of, for example, borax (sodium tetraborate), preferably in the anhydrous form, and carbon is introduced through line 50 to a charging hopper 52. Electrically conductive balls are provided to hopper 52 via a chute 54 at the upper end of an enclosed elevator 56, preferably of a bucket-type. The charge from the hopper 52 is metered into the top of a refractory walled furnace reactor 58 through a rotary star valve 60. The relative amount of borax to the total Weight of reducing carbon and electrically conductive balls in the bed is kept relatively low, typically in the range of 5 to 25% by weight, thus assuring that the rnolten borax will not fuse the bed. The electrically conductive particles may be formed of petroleum coke or other compacted carbon or alternatively of another electrically conductive material such as sintered boron carbide.

The electrically conductive balls are removed from the bottom of the furnace reactor 58 through a gas-solid lock 62, which is provided at its respective upper and lower ends with star valves 64 and 66. The star valves 60, 64 and 66 are of the familiar rotary type which are capable of metering solid material therethrough at a constant rate and still provide a substantial gas seal. A small amount 11 of stripping gas may be introduced to the lower end of the furnace reactor 58 through lines 68, thus insuring the removal of substantially all of the sodium vapors from the furnace reactor through line 70.

The furnace reactor 58, intermediate of its length, is provided with electrodes 72 through which electrical energy is supplied to the process. The electrodes 72 are positioned within a central heating section 73 of the furnace reactor 58. The refractory walls of the furnace above the heating section 73 taper downwardly and outwardly. The refractory walls of the furnace reactor 58 below the heating section 73 taper inwardly and downwardly to the star valve 64. The expanded central heating zone 73 provides the desired hold-up time within the reactor for the heating of the furnace content to achieve substantial reduction of the reducible alkali metal compound. The outwardly and downwardly flaring walls above the central zone minimize the possibility of material blockage while the inwardly-downwardly flaring of the walls below the central zone funnels the processed material to a central removal area.

The lower end of the lock 62 opens onto a shaker screen 74 which is sized to retain the electrically conductive balls and permit the passage therethrough of the boron and boron carbide product. From the screen 74 the electrically conductive balls pass via line 75 to the bottom of the aforementioned conveyor 56 from whence they are raised to the charging hopper 52. In the event there is a build up of material on the balls, a portion of them are periodically withdrawn and tumbled for removal of accumulated material. The reduced material, boron and/or boron carbide fines in the instant example, which have passed through the screen 74 are available for processing or further use and are removed via line 77.

The remainder of the flow diagram FIG. 2 is substantially like that of the process illustrated in FIG. 1 and will be seen to comprise a spray condenser 78 into which the vapor line 70 opens. In the condenser 78 a stable highboling hydrocarbon oil is sprayed downwardly into the sodium-carbon monoxide gas stream. The vapors are quickly cooled to about 250 C. or somewhat less, whereupon the sodium liquefies and adds to the liquid of the pool. The temperature of the liquid pool of oil and alkali metal may be regulated by circulation through pump 80 and cooler 82. The gases, including any volatized oil, leave the spray condenser 78 through line 79 for cooling in an overhead condenser 84, from which a portion of the gases may be recycled to the furnace reactor 58. Makeup oil is supplied to the spray condenser via line 86. The condensate recovered in the overhead condenser 84 drains into the spray condenser 78 therebelow.

Liquid from the spray condenser 78 is removed via line 88 to a heated basket centrifuge 90 which has its inner surface coated with a petroleum coke. A petroleum coke slurry is added to line 88 immediately preceding the basket centrifuge 90 via line 92. The liquid stream from the spray condenser 78 enters the interior of the basket and passes through the walls thereof, with the solid contaminants being filtered out and retained. These solids which will comprise principally carbon and sodium carbonate are withdrawn from central bottom of the basket centrifuge 90 and collected in a bin 94, from whence they may be removed via conveyor 96 and eventually returned to the furnace reactor. The liquid from the centrifuge 90 is made up of hydrocarbon oil and liquid sodium and is removed through line 98 to a vessel 100. The oil-liquid sodium mixture within the vessel 100 is further cooled by circulation through a loop 101 which contains heat exchanger 102. Liquid from the loop 101 is partially withdrawn through line 104 to a solid bowl centrifuge 106 where the stream divides into a metallic sodium product stream 108 and the earlier mentioned hydrocarbon oil stream of line 86 through which the oil returns to the spray condenser 78.

The process is directed to the processing of those alkali metal compounds which at the temperature of the carbothermic reduction are in a melted state, thus presenting a severe material-handling problem. The alkali metal compounds proccssable in the method of the invention are further characterized in providing upon reduction either a solid, electrically-conductive residue (as in the instance of borax) or, alternatively, substantially no bed residue (as in the case of sodium carbonate). Heretofore, the carbothermic reduction of alkali metal compounds of the foregoing description have presented some very difficult technological problems.

The carbothermic reduction of such alkali metal c0mpounds requires a large input of heat and the reactions proceed only at high temperature. For example, the car- 'bothermic reduction of sodium carbonate may be carried on at a temperature as high as 1500 C. The alkali metal compounds to which the process of the invention is adaptable, melt before the reduction is effected. It will be appreciated that the mere mechanical handling of such hightemperature corrosive masses presents diflicult problems. With the process of the invention, alkali metals may be produced at high yields and with high elficiencies by utilizing a downwardly-moving bed made up predominantly of electrically-conductive, carbon-containing balls (for example, densified carbon or a solid carbide material such as boron carbide or silicon carbide) through a heating zone of the carbothermic furnace. The reducible alkali metal compound being processed and a carbon reducing material, usually petroleum coke, are dispersed in the bed in particles smaller than the electrically-conductive balls. The reducible alkali metal compound comprises on a weight basis less than 50% of the weight of the bed. With the foregoing described bed, it is possible to handle the alkali metal compound despite its lower melting temperature, without the bed forming a highly-corrosive, high-temperature fused mass, which would, in the absence of the advantages of the bed, be substantially impossible to handle on a continuous basis.

In providing, as in the preferred process, the electrically-conductive balls in a predominant amount and larger in size than the alkali metal compound particles being reduced, the melted alkali metal compound is dispersed in the larger amount of balls and the balls, in a sense, pr0- vide a vehicle for the downward passage of the reactants through the high temperature reduction zone of the furnace, along with conducting the electrical current for heating the bed. Upon the reduction of the alkali metal compound, there is no longer a fused residue interfering with the operation of the bed. For example, in the processing of sodium carbonate, there is substantially no bed residue, and the elemental sodium is removed in a vapor stream from the bed and furnace. In the instance of borax and other alkali metal-amphoteric metal compounds, the alkali metal vapors are removed overhead and there remains a solid residue of reduced boron and boron carbide particles which are even more electrically-conductive than carbon.

The electrically-conductive balls are withdrawn from the base of the furnace and recycled to make up successive strata of the downwardly-moving bed. The withdrawn balls are preferably not cooled before recycling, since the recycling of the hot electrically-conductive balls improves greatly the efliciency of the process.

Although exemplary embodiments of the invention have been disclosed herein for purposes of illustration, it will be understood that various changes, modifications, and substitutions may be incorporated in such embodiments without departing from the spirit of the invention as defined by the claims which follow:

1. A method for producing elemental alkali metal from a reducible alkali metal compound in a carbothermic reduction furnace, said method comprising:

passing a downwardly moving bed made up predominantly of electrically-conductive carbon-containing balls through a heating zone of the furnace with the reducible alkali metal compound and a carbon reducing material dispersed in the bed in particles smaller than said balls, said reducible alkali metal compound comprising on a weight basis less than 50% of the bed;

said reducible alkali metal compound being selected from the group consisting of an alkali metal carbonate, an alkali metal hydroxide, and an alkali metal compound having a negative group containing an amphoteric metal and with said reducible alkali metal compound being further characterized in melting at the temperature of the heating zone and in the instance of the foregoing alkali metal-amphoteric metal compound, providing upon reduction the alkali metal as vapor and the amphoteric metal in a reduced, electrically-conductive, solid form;

heating the bed within said zone by passing an electric current therethrough and raising the temperature of the bed therein to the range of about 900 C. to about 1500 C. and at a temperature less than that causing significant volatization of the alkali metal compound and causing a melting of the alkali metal compound;

eflecting a reduction of the melted alkali metal compound to produce alkali metal vapors, said melted alkali metal compound upon reduction leaving no fused residue and leaving substantially no residue where the alkali metal compound is a carbonate or a hydroxide and in the instance of the alkali metalamphoteric metal compound leaving a bed residue comprised of the amphoteric metal in a reduced, electrically-conductive, solid form;

removing the alkali metal vapors in a gaseous stream from the bed and furnace and recovering the alkali metal therefrom; and

withdrawing electrically-conductive balls from the base of the furnace and recycling the withdrawn balls to make up succeeding strata of the downwardly moving bed.

2. A method in accordance with claim 1 wherein the electrically-conductive balls are formed of carbon or a solid carbide material.

3. A process in accordance with claim 1 wherein the 14 carbon reducing material is supplied to the bed in at least a stoichiometric amount for reaction with the alkali metal compound.

4. A process in accordance with claim 1 wherein the alkali metal material is anhydrous borax and the solid, electrically-conductive residue is a reduced boron material.

5. A process in accordance with claim 1 wherein the electrically-conductive balls typically have a dimension of A to 3 inches.

6. A process in accordance with claim 1 wherein the reducible alkali metal compound comprises on a weight basis 5 to 30% of the bed.

7. A process in accordance with claim 1 wherein the solid residue in the instance of an alkali metal-amphoteric metal compound is separated from the withdrawn electrically-conductive balls and the balls then recycled to the furnace.

8. A process in accordance with claim 1 wherein the reducible alkali metal material is selected from the group consisting of sodium carbonate, sodium hydroxide, sodium metaborate, sodium tetraborate, sodium silicate, sodium vanadate, sodium tungstate, and sodium chromate.

9. A process in accordance with claim 1 wherein the carbon reducing material is petroleum coke.

References Cited UNITED STATES PATENTS 2,920,951 1/1960 Bretschneider et a1. 2,810,636 10/1957 Kirk --66 2,379,888 7/ 1945 Doerner.

673,761 5/1901 Cowles. 3,449,116 6/1969 Derham 75-63 3,449,117 6/1969 Derham 75--63 920,473 5 1909 Johnson. 2,978,315 4/ 1961 Schenck. 1,901,525 3/ 1933 Moschel.

HENRY W. TARRING II, Primary Examiner U.S. Cl. X.R. 75-10 A, 26 

